In operation such fans may be used to blow, or alternatively draw, air through a heat exchanger such as a radiator. In either case it is desirable to provide a high efficiency fan to avoid waste of energy.
One source of energy loss during operation of axial flow fans is the so-called "re-flux" of air around the axial periphery of the fan caused by air flowing from the high pressure side of the fan to the lower pressure side. This air flow is also known as a tip vortex and is not useful because the fan is using energy to create air flow which does not flow through the heat exchanger. Moreover this air flow creates noise which is itself both inefficient and can also be intrusive and unpleasant.
Many solutions have been proposed for the solution of these problems. For example in U.S. Pat. No. 4,566,852 (Hauser) a cooling arrangement using an axial flow fan is provided which improves the efficiency of the fan and reduces noise level during operation. This is achieved by providing an air guide structure in the form of a ring attached to the fan blades which cooperates with a non-rotating fan frame (or shroud) so that the contours of the air guide conforms closely to the facing contour of the shroud thereby providing a constricted path for re-flux air flow to reduce it. Additionally the re-flux air flow (known in that reference as "slot air flow") is strongly throttled as a result of the shape of the air guide and its close cooperation with the shroud, and this leads to a reduction in noise.
However in such an arrangement the additional mass of the rotating ring may create mechanical stress. In addition the provision of the rotating ring increases material costs in manufacturing the fan.